Offshore exploratory drilling is usually done using self-contained bottom supported platforms and floating drilling platforms, or rigs, that can be easily moved. One of the most common types of bottom-supported rigs are the jack-up rigs wherein the legs of the rig are permanently attached to the mat which is jacked up and down in relation to the platform. The term drilling platform and rig will be used interchangeably herein. One type of jackup rig is the independent jack-up rig which is towed to location with its legs elevated. On location, the legs are lowered to the bottom and the platform is "jacked up" above the wave action by means of hydraulic jacks or electric motor rack and pinion equipment which are located within the platform structure, typically in a so-called pod house.
Another type of commonly used jack-up rig is the so called mat supported jack-up rig, which is typically used for soft bottom conditions. The mat supported jack-up rig is comprised of a base mat to which a plurality of legs are attached, and a working platform which may be moved up and down on the legs with respect to the base mat, but above the wave action. The mat is typically a hollow oblong structure, preferably rectangular in shape, and of appropriate dimensions for providing stability for the rig when resting on the seafloor. The mat structure usually contains one or more cutouts for such things as allowing access to the seafloor for drilling, and for helping to relieve any suction that may have developed under the mat, thereby making it easier to raise the mat from the seafloor when relocating the rig.
The interior of the mat typically contains compartments, such as ballast compartments, which can be judiciously flooded to the appropriate degree to make the mat neutrally buoyant. The mat is made neutrally buoyant on conventional mat supported jack-up rigs for greater stability for both towing and for when the mat is in its normally seafloor position on location. For example, if the mat of a conventional jack-up rig had a positive buoyancy, it would exert too much upward force on the platform during towing, thus making it unstable and unseaworthy. On the other hand, if the mat were less than neutrally buoyant, it would tend to drag the platform underwater during towing, also making it unseaworthy. Once moved into position, the neutrally buoyant mat is lowered and raised more easily than if were not neutrally buoyant.
While bottom supported rigs, such as the jack-up rig, can be used in most locations where the water depth is less than about 200 feet, preferably less than about 150 feet, there are still some locations, even in relatively shallow waters, where bottom supported rigs are not desirable. For example, in some locations, such as at mouths of rivers, and in some soft bottom lakes, neither the mat supported jack-up rig nor the independent jack-up rig are suitable for use. For example, the bottom conditions may be too soft to support the independent jack-up rig, and the mat supported jack-up rig may pose too great a risk at locations which have been so extensively developed that a network of seafloor pipelines must be protected in the vicinity of the proposed rig. While it would be desirable to use floating rigs in such locations, some floating rigs present their own set of problems. For example, cantilever type floating rigs, which may be successfully used in the drilling operation, may not be readily adaptable to a workover role, which typically requires more exacting positioning between the rig and the well. Generally, there is less tolerance for vertical movement of the drillfloor/drillstring assembly in workover operations. Any wind, swell, current, wave action, and the like, imparts a magnified amplitude motion vertically at the wellhead, which presents both safety and operational problems. Thus, there is a need in the art for an economical floating rig having improved stability which can be used in such locations and which can compensate for, or minimize, motion due to wave action, etc.